FN is a Mauser 98 large ring as has already been stated. Bell and Carlson, Manners, McMillan and many others make stocks for this. Figure out what you're wanting out of the stock (ie., light weight, better bedding, fit, feel etc) then research what you want.
McMillan does some that folks call 'McSwirly's' that have two or more colors blended in. The McMillan's and Manners aren't cheap - $400-$600 and there's quite a wait on them, but folks swear by them.